Recently, light emitting diodes (LEDs) have come to be widely used in a variety of types of electrical equipment, for example, display devices, optical disk apparatuses, and the like, because of their low power requirements and long life. The light emitting diode emits light by supplying an electrical current to the light emitting diode by a LED drive circuit.
The LED drive circuit generally employs a so-called LED-terminal-voltage-comparison method because it is possible to reduce total power consumption of the LED drive circuit in light of fluctuations in forward voltage of the LED caused by inconsistencies in the manufacturing process. In the LED-terminal-voltage-comparison method, a switching regulator that forms a constant voltage circuit to supply power to the LED is controlled in accordance with results of a comparison made between a voltage that is a terminal voltage between LED terminals and a reference voltage.
However, when lighting the LED is controlled in accordance with a pulse signal modulated by pulse width modulation (PWM) using this drive circuit, it is not possible to control the terminal voltage of the LED stably, especially when the LED is on. Therefore, the only alternative is to employ a so-called output-voltage-comparison method, in which an output voltage of the switching regulator is controlled in accordance with a voltage comparison result of a divided voltage obtained by dividing the output voltage of the switching regulator with respect to a predetermined reference voltage.
FIG. 1 is a schematic diagram of a circuit configuration of a known LED drive circuit. In FIG. 1, the LED drive circuit 100 includes a constant voltage circuit 101, a constant current circuit 102, and a reference voltage generator 103. The constant voltage circuit 101 outputs a predetermined constant voltage to a LED 110. The constant current circuit 102 supplies a constant current to the LED 110. The constant voltage circuit 101 forms a switching regulator and controls a switching transistor (not shown) to supply a predetermined constant voltage to the LED 110, so that a divided voltage obtained by dividing an output voltage of the constant voltage circuit 101 becomes a predetermined reference voltage Vref output from the reference voltage generator 103. A pulse signal Spwm is input to the constant current circuit 102 externally, and the constant current circuit 102 supplies a constant current to the LED 110 in accordance with the pulse signal Spwm.
However, in the LED drive circuit 100, the reference voltage must be determined in consideration of fluctuations in forward voltage of the LED caused by inconsistencies in the manufacturing process. Accordingly, the total power consumption of the LED drive circuit increases compared to the LED-terminal-voltage-comparison method previously described.
A typical forward voltage value of a white LED used for a display of a portable device is 3.2 volts (TYP), a minimum voltage value (MIN) is 3.0 volts, and a maximum voltage value (MAX) is 3.9 volts. Therefore, in the output-voltage-comparison method, the reference voltage Vref must be determined in consideration of the MAX voltage value, 3.9 volts which is the maximum voltage value of the forward voltage of the white LED due to the variation caused during the manufacturing process. Since the typical voltage value of the forward voltage of the white LED is 3.2 volts, the drive circuit needs to output a voltage for most of the LED devices that is higher by 0.7 volts than an output voltage for the typical LED device. When a higher output voltage is output, the power consumption of the total drive circuit increases. Further, large power consumption may cause a heat problem when the drive circuit is formed in a semiconductor integrated circuit.